Donna Gregory sat in her apartment and screamed at her neighbor in the adjoining unit.

Her neighbor sat in her apartment and screamed back.

This is what they found out: Neither could hear the other.

At that moment, fortunately, neither needed the other. They just wanted to see if they could, in case the need arose.

Donna Gregory, a registered nurse and director of Nursing and Health Services for the Fairfax County Red Cross, had begun to think seriously about how people who live alone can get help:

She had become dangerously ill inside her apartment. A co-worker at the Red Cross told her to be sure and leave her door unlocked in case she couldn't get up when her friend got there. Gregory, confused by her illness and forgetting she had already unlocked her door, got up and relocked it. She was able to get up when her friend came, as it turned out, but the near miss was disconcerting.

Although Gregory had a first-rate support system, it occurred to her many people do not. Moreover, as a health professional she knew the necessities to have on hand in case of an illness or accident. Again, she speculated, many do not.

Over the past year or so, Gregory and Marcie B. Barnard, a specialist in community-health education, have developed a plan for a one-time, two-hour course aimed initially at teaching self-help to singles, but applicable to anyone alone for even relatively short periods.

The course -- "Develop Your Personal Health Support System: A Guide For Those Who Live Alone" -- is in part designed to help people living away from their families set up new support systems. They could be with neighbors or colleagues, but should include at least one "key" support person -- a sort of surrogate parent -- who could water flowers, feed pets, make sure children are cared for, see that an abandoned car is driven home or even that the phone bill is paid.

The course and its syllabus guide -- much of which will be available to participants -- is a compendium of common-sense precautions, preparedness tips, useful lists, charts and instructions. For example, what do you do when you are choking and no one is around to perform the Heimlich maneuver? (Answer: "Press your fist between your rib cage and waist with a quick, inward and upward thrust, or lean forward and press your abdomen quickly over any firm object, such as the back of a chair or a porch railing.")

Organized into four basic categories -- psychological, financial, environmental and personal -- the guide addresses both the value of taking responsibility for one's own health and the necessity of recognizing when outside help is needed.

* Psychological: "A lot of people," says Barnard, "are afraid to go out for help. But there are places to go when you need help and you should be able to seek it. Nobody's going to come to you when you're sick. You have to make the first effort."

* Financial: "When I begin to feel sick," says Gregory, "I stop at the bank on the way home for some money. . .you need cash. Suppose you need to take a cab? Get a prescription?" Gregory believes people should have enough money available to pay bills for three months and have someone lined up to do the paying -- in case. The course also includes discussions of community resources, insurance, disability, longterm arrangements for incoming funds.

* Environmental: After she stops at the bank, Gregory (going home with something like the flu) also picks up a pint of ice cream. "If I'm going to have a temperature of 101 degrees, I'm not going to want to cook . . ." She also recommends having available easy-to-prepare and nutritious food such as soups, Jell-O, juices, tea and crackers.

The course distributes and encourages using Vials of Life--glass vials kept in the refrigerator--which contain a brief medical history for each resident. A Vial of Life decal on the refrigerator door alerts emergency medical personnel.

Personal: To help individuals develop a support system, Gregory and Barnard schedule what they call a "trust experience." Blindfolded participants are led around by strangers, to "feel yourself," says Barnard, "relying on someone else."

"Even if people don't take the course," says Barnard, "they could do just one thing: Talk to a neighbor and ask if you can call them if you get sick and if they would notify someone else . . .and suggest you'd be willing to do the same."

Although thinking about sickness is not the most pleasant occupation, "think of it this way," says Gregory, "it's a great way to meet new people."

Fairfax County Chapter of the American Red Cross courses: at 7:30 p.m. Sept. 22, Sept. 29 and Oct. 6 at chapter headquarters, 4117 Chain Bridge Rd., Fairfax, Va. Phone: 591-8091. Tuition is $5.